A ship's move is calculated by adding the full move of all seafarer units designated as crew, adding 1 move per each non-seafarer unit designated as crew, and dividing that total by some number determined by the condition and design of the ship.

Ships attack with Beam weapons, whose strength is calculated in a similar manner as its move.

Class

Boarding

Ship's crew can try to grapple an enemy vessel so that units can enter it to capture or destroy it. Boarding or defending units don't have to have seafarer special or be part of the ship's crew.WB2014 Duke Forecastle - Part 12

Speculation

There may be different types of ships, or the type may change with levels. One type is the Galleon.

Ships may level up via experience earned in combat like other units, or they may have to be created at a particular level, or increase in some other way.

Ships may level by by being upgraded like cities.

Carrying Capacity

A Lvl 3 Haggar ship carried twenty four Infantry Units, while two Level 2s and a Level 4 carried a total of seventy five Infantry, seven small Flyers, one Heavy Flyer, and a Warlord.LIAB Text 18
One possibility is limits of 10 units per level.